The traditional version of a pliers includes two elongated members joined at a pivot pin. One end of each elongated member forms a jaw, and the other end forms a handle. Workpieces of different sizes are grasped in different manners, due to the constant geometry of the elongated members and the jaws. Some adjustability may be achieved by providing a slotted receiver in one of the handles, so that the handle with the pivot pin may be moved between different positions in the slot to provide adjustability for gripping objects of different sizes.
U.S. Pat. No. 4,651,598 provides an improved pliers whose jaws are self adjusting according to the size of the workpiece. Commercial versions of this pliers are useful, but have significant drawbacks. Perhaps the most significant problem with the pliers made according to the '598 patent is that the jaws move slightly relative to each other in an end-to-end manner as they are clamped down onto a workpiece. The surfaces of soft workpieces such as brass or copper may be marred as a result. The clamping force applied by these pliers depends upon the size of the workpiece being grasped.
Another problem with the pliers of the '598 patent is that they do not lock to the workpiece, an important convenience in some uses of pliers. Overcenter locking pliers are described in a series of patents such as U.S. Pat. No. 4,541,312. Conventional overcenter locking pliers provide adjustability in the size of the workpiece that may be gripped through a screw adjustment to the pivoting position of the control arm, but this adjustability is not automatic in the sense of the pliers of the '598 patent.
Other types of locking pliers such as the AutoLock™ pliers combine the self-adjusting feature with an overcenter locking mechanism. This pliers can be inconvenient to use for some sizes of workpieces, suffers from some of the problems of the pliers of the '598 patent, does not achieve a large gripping force, and may unexpectedly unlock when large objects are being gripped. Additionally, as with some other pliers, two hands are required for its operation.
There is a need for a self-adjusting pliers which does not experience shifting of the jaw position as the object is grasped, which may be operated with one hand, and which may be provided in a locking version. There also exists a need for a self-adjusting pliers where one hand may operate and adjust the force that is applied to a workpiece grasped between a first and second jaw. The present invention fulfills these needs.